The compound, 3,5-difluoroaniline is a key intermediate in the synthesis of many broadleaf herbicides and other agricultural chemical and pharmaceutical products. Many synthetic routes to this compound have been investigated. However many such syntheses are difficult, or do not render a sufficient yield, or are simply too costly. For example, it is difficult to introduce two fluorine substituents in the 3 and 5 positions relative to amino- or nitro-functionality. For this reason, technically elaborate and expensive synthesis alternatives have been attempted. However, unfavorable yields and an abundance of interfering reaction byproducts make such syntheses impractical. In the realm of agricultural chemicals, cost studies have repeatedly indicated that only short synthetic pathways (3 steps or less) are economically feasible.
For example, syntheses displacing chloride by fluoride in 1,3,5-trichlorobenzene are known. However, the reaction time was slow with unacceptably low yields. R. G. Pews, in J. Fluorine Chem. Vol. 52, page 307 (1991), has suggested that faster reaction rates and higher yields could be obtained using an autoclave and different solvents. However when the reaction was run in N-methylpyrrolidone (NMP) using CsF and KHF.sub.2 as catalysts, yields were not as high as those reported by Pews. Further, the use of NMP caused substantial halogen reduction.
U.S. Pat. No. 5,294,742 disclosed preparation methods for 3,5-difluoroaniline where 2,4,5-trichloronitrobenzene is used as a starting material and reacted with an alkali metal fluoride. However, the synthesis is complicated due to the multiple reaction steps, and the high quantities of by-product (20% 2,6-difluoroaniline) produced.
U.S. Pat. No. 5,399,767 discloses another complicated method comprising reacting a benzonitrile with a mineral acid, decarboxylizing to an intermediate, and reacting with hydrogen to make 3,5-difluoroaniline.
In view of the significant commercial use of 3,5-difluoroaniline, there has been substantial research directed to identifying commercially acceptable, safe, inexpensive and efficient methods for its manufacture in suitably high yields. However, there is no present method known for producing 3,5-difluoroaniline in high yields with few by-products to be separated, that is also safe and inexpensive.